In order to be able to achieve synchronization throughout a network, it is known to transmit timing information between network nodes. One example of a situation where this applies is the case of a cellular communications network, where it is necessary to achieve synchronization between the access points in the respective cells. One method of transmitting the timing information uses the Precision Time Protocol (PTP), and requires a node to measure a time at which it receives a message, and to measure a time at which it forwards that message on to a destination. The elapsed time between the two times is referred to as a residence time. Information about this residence time is sent to a destination node, for use in calculating transmission delays over the network, and thus for use in achieving synchronization between the nodes of the network.
Multiprotocol Label Switching (MPLS) is a mechanism used for transporting data packets across networks, and the document “Residence Time Measurement in MPLS network”, Mirsky, et al, available at http://www.ietf.org/internet-drafts/draft-mirsky-mpls-residence-time-02.txt, describes a system in which Residence Time Measurement information can be transmitted in a Generic Associated Channel (G-ACh) message. Specifically, this document describes a system in which a data packet includes information indicating the residence time spent in routers transited by the packet on its path from an ingress router to an egress router.
However, this system places certain requirements on the hardware, and it may not be possible to meet these requirements in all cases.